a) Field of the Invention
This application relates to an electrophoretic display with improved contrast ratio, switching performance, reflectivity at the Dmin state and structural integrity, and methods for its manufacture.
b) Description of Related Art
The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
In order to prevent undesired movement of the particles, such as sedimentation, partitions between the two electrodes were proposed for dividing the space into smaller cells (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152(1979)). However, in the case of partition-type electrophoretic displays, difficulties were encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it was also difficult to keep suspensions of different colors separate from each other in the partition-type electrophoretic display.
Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs (the channel or groove type). The filling and sealing of electrophoretic fluid in the channels are accomplished by a batch-wise process. In addition, the problem of undesirable particle movement or sedimentation, particularly in the longitude direction, remains an issue.
Subsequently, attempts were made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804, 5,930,026 and 6,017,584 describe microencapsulated electrophoretic displays. The reference display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as physical processes, in-liquid curing or simple/complex coacervation. The microcapsules, after their formation, may be injected into a cell housing two spaced-apart electrodes, or “printed” onto or coated on a transparent conductor film. The microcapsules may also be immobilized within a transparent matrix or binder that is itself sandwiched between the two electrodes.
The electrophoretic displays prepared by these processes, in particular, the microencapsulation process as disclosed in U.S. Pat. Nos. 5,961,804, 5,930,026 and 6,017,584, have many shortcomings. For example, the electrophoretic display manufactured by the microencapsulation process suffers from sensitivity to environmental changes (in particular, sensitivity to moisture and temperature) due to the wall chemistry of the microcapsules. Secondly, the electrophoretic display based on the microcapsules has poor scratch resistance due to the thin wall and large particle size of the microcapsules. To improve the handleability of the display, microcapsules are embedded in a large quantity of polymer matrix which results in a slow response time due to the large distance between the two electrodes and a low contrast ratio due to the low payload of pigment particles. It is also difficult to increase the surface charge density on the pigment particles because charge-controlling agents tend to diffuse to the water/oil interface during the microencapsulation process. The low charge density or zeta potential of the pigment particles in the microcapsules also results in a slow response rate. Furthermore, because of the large particle size and broad size distribution of the microcapsules, the electrophoretic display of this type has poor resolution and addressability for color applications.
An improved EPD technology was recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488 filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/606,654 filed on Jun. 28, 2000 (corresponding to WO02/01280) and U.S. Ser. No. 09/784,972 filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference in their entirety. The improved EPD comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles or pigment-containing microparticles dispersed in a dielectric solvent or solvent mixture, preferably a fluorinated solvent or solvent mixture. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
The microcup structure enables a format flexible and efficient roll-to-roll continuous manufacturing process for the preparation of EPDs. The displays can be prepared on a continuous web of a conductor film such as ITO/PET by, for example, (1) coating a radiation curable composition onto the ITO/PET film, (2) forming the microcup structure by a microembossing or photolithographic method, (3) filling an electrophoretic fluid into the microcups and sealing the filled microcups, (4) laminating the sealed microcups with the other conductor film and (5) slicing and cutting the display to a desirable size or format for assembling.
One advantage of this type of EPD is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of microcup displays are significantly better than any displays previously known including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut to almost any dimensions without the risk of damaging the display performance due to the loss of the display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of display panel in a large sheet format which can be cut into any desired format. The isolated microcup or cell structure is particularly important when cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing or being subject to cross-talk during operation.
In order to achieve a higher contrast ratio, one of two approaches may be taken: (1) using a darkened background to reduce the light leaking through the inactive partition wall or (2) using microcups of wider opening and narrower partition to increase the payload. However, the darkened background typically results in a lower reflectivity at the Dmin state. On the other hand, display cells formed from wider microcups and narrower partition walls tend to have a poor resistance against compression and/or shear forces imposed by, for example, a sharp stylus for a touch screen panel.